It is well known that golf balls are typically constructed with a cover that tightly surrounds a core. It is typical for a golf ball core to have a solid construction or a wound construction and the methods of forming these cores are well known in the art. Traditionally, golf ball covers are formed from polymeric materials. For instance, golf balls have traditionally incorporated covers made of balata rubber, which may be a natural balata, a synthetic balata, or a blend of natural and synthetic balata.
Other golf balls have incorporated covers which are formed from synthetic polymeric materials such as polyolefins and in particular, polyethylene, polyurethanes, and ionic copolymers of olefins. The latter mentioned ionic copolymers of olefins were commercially introduced in the mid 1960's by E. I. Du Pont de Nemours & Co., Inc., Wilmington, Del. (DuPont) and sold under the trademark “SURLYN.” Golf balls incorporating SURLYN covers are generally described in U.S. Pat. No. 3,454,280. Cover compositions that are based on SURLYN resins are advantageous in that the resulting covers are cut and abrasion resistant compared to the balata covers. While golf balls incorporating SURLYN resin covers are commonly known by players to be more cut resistant than balata covered balls, they traditionally tend to reduce the spin imparted to a golf ball and produce a less desirable “feel” as compared to a balata covered ball.
SURLYN resins sold by DuPont typically contain zinc, lithium, magnesium or sodium ions. A number of SURLYN resins, of varying physical properties, are sold by DuPont. The physical properties of these resins are described in technical bulletins that are readily available from DuPont. Mixtures of various SURLYN resins as cover stock materials are likewise highly advantageous. Suitable mixtures for use as cover materials are described in U.S. Pat. No. 3,819,768.
For purposes of control, golfers strike a golf ball in such a manner that the ball has substantial backspin. It is desirable that a golfer be able to impart backspin to a golf ball for purposes of controlling its flight and controlling the action of the ball upon landing on the ground. For example, substantial backspin will make the ball stop once it strikes the landing surface instead of bounding forward. The ability to impart back spin onto a golf ball is related to the deformation of the golf ball cover when struck with a golf club. Generally, the more deformable the cover is, the easier it is to impart spin to the balls. This is particularly true for short or wedge shots.
Thus, it is desirable to combine the properties of SURLYN covered golf balls with the properties of Balata covered golf balls. For example it is desirable to have less spin on a drive, such that the ball will have a “low spin trajectory”. The result is that the ball does not climb like a typical high spin rate ball would, and the ball has substantial roll after it lands on the ground to provide maximum distance. On the other hand, for approach shots, i.e., short shots into the green, spin is critical to control the ball when it lands. With a high spin rate, the ball will stop or “sit” when it hits the green. Thus, with a high spin rate, the ball can be hit directly at the target. With a low spin rate, the ball often bounces off the green or “runs” off the green. Thus, it is desirable to have a high spin rate for approach shots into the green.
Further, it is desirable to combine the durability of SURLYN covered balls with the characteristics of balata covered balls.
Typically, the golf ball cover layer is formed by one of two processes. The first process includes the compression molding of hemispheres. First, two hemispherical covers, called half-shells, are injection molded. The hemispheres are then placed around a core and compression molded so that they fuse around the core and wherein dimples are imparted into the cover. The cover is then finished to remove any visible molding lines or residue. The second process, called the retractable pin injection molding process, involves injection molding of the cover directly around a core positioned on pins, removing the pins once the cover material surrounds the core, removing the covered core, and finishing it to form a completed golf ball. In both cover forming processes, the injection molding of the covers involves techniques known in the art. These techniques generally involve forcing molten material to substantially fill and take on the shape of a mold, thereby forming a cover or hemisphere. When the material is cool enough to substantially maintain the shape of the mold, it is ejected from the mold.
Typically, the cover material begins the injection molding process as resin pellets that are stored in a hopper. The pellets are gravity fed into a heated cylinder that melts the pellets as a screw simultaneously pushes the softening pellets toward an accumulation zone. When enough molten material is accumulated to fill the mold, the screw is pushed or stroked forward, thereby forcing the melted material into the mold. Many prior art references are directed to mixing different materials to form new cover materials. This is traditionally accomplished by mixing pellets of different materials in the injection mold machine hopper.
There are also many patents that are directed to golf balls having multiple cover layers. For example, U.S. Pat. No. 4,431,193 relates to a golf ball having a multilayer cover wherein the inner layer is a hard, high flexural modulus ionomer resin and the outer layer is a soft, low flexural modulus ionomer resin, and wherein either or both layers may comprise a foamed ionomer resin.
U.S. Pat. No. 5,314,187 also relates to golf balls having multiple layer covers, wherein the outer layer is molded over the inner layer and comprises a blend of balata and an elastomer where the inner layer is an ionomer resin.
U.S. Pat. No. 4,919,434 is directed towards a golf ball having a cover which comprises an inner layer and an outer layer each of which is a thermoplastic resin. Preferably the layers comprise thermoplastic resin materials that are capable of fusion bonding with each other.
U.S. Pat. No. 5,783,293 discloses a golf ball with a multi-layered cover formed by a co-injection molding process, wherein the golf ball cover comprises an inner and outer layer of a first material and an intermediate layer therebetween of a second material.